Add a pre-check for pingbacks, to stop spam before an outbound verification request is made.

There was a news cycle a few days ago about “WordPress pingbacks being used to DDOS sites” which had a lot of misinformation and hyperbole, but there were two valid issues which the last two bullet points address: anti-spam checks were done after a pingback was verified, and WP didn’t pass on who made the request that caused it to verify a pingback (effectively cloaking the true source). This update to Akismet addresses both, and we think a similar approach may be appropriate for core in a future release.